1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus such as an electrophotographic apparatus capable of almost simultaneously carrying out processes such as charging, exposure, development and cleaning without depending on a corona discharge.
2. Description of the Prior Art
As electrophotographic apparatuses generally known include one utilizing a corona discharge. Such an electrophotographic apparatus has a disadvantage of having complicated structure because components for carrying out processes such as a corona discharge, exposure, development, transfer, elimination of charge and cleaning must be arranged around a photoreceptor.
In view of the above described disadvantage, there has been proposed in recent years an electrophotographic apparatus using an electrophotographic process which utilizes no corona discharge, that is, an internal illuminating type or charge injection type electrophotographic process in, for example, an article entitled "Photoreceptor charging mechanism by conductive particle rubbing and application to a novel electrophotographic printing technology", J. Appl. Phys. 63.(11), Jun. 1, 1988.
FIG. 7 is a schematic diagram showing structure of this newly proposed image forming apparatus. The image forming apparatus is constructed by arranging a developing device 12 above a photoreceptor 10 and a transferring device 14 below the photoreceptor 10 as well as arranging an LED array head 16 inside the photoreceptor 10. More specifically, the photoreceptor 10 is constructed by laminating a transparent electrode 10b and a photoconductive layer 10c which constitutes a photosensitive member on an outer periphery of a cylindrical transparent substrate 10a made of a glass. A voltage (V) of approximately 20 volts is applied as a developing bias between the transparent electrode 10b, that is, the photoconductive layer 10c and a magnetic roller 18 constituting the developing device 12, that is, a developing sleeve 20. A conductive magnetic toner 22 is absorbed on a periphery of the developing sleeve 20 covering an outer periphery of the magnetic roller 18, and a so-called magnetic brush is formed. The magnetic brush faces the outer peripheral surface of the photoconductive layer 10c. An electric charge is injected into the photoconductive layer 10c from the developing bias through the conductive magnetic toner 22 so that the photoconductive layer 10c is charged to approximately the same potential as the developing bias.
On the other hand, if an exposure light projected from the LED array head 16 is incident on the photoconductive layer 10c from an inside of the cylindrical transparent substrate 10a to form an electrostatic latent image on the photoconductive layer 10c, the toner 22 is adhered on the surface of the photoconductive layer 10b from the magnetic brush, and therefore, a toner image is formed. The toner image is transferred onto a recording paper 24 by the transferring device 14. Remaining toners on the surface of the photoreceptor 10 are removed by a cleaning force of the developing device 12 and a magnetic force of the magnetic roller 18. Consequently, processes such as charging, exposure, development and cleaning are almost simultaneously carried out by the developing device 12 and the LED array head 16, and therefore, structure of an electrophotographic apparatus as well as electrophotographic process can be significantly simplified.
In the above described method where the conductive magnetic toner 22 is utilized, in a case of a direct transferring system, it is required to use a high-resistance recording paper which is obtained by coating a specific material onto a plain paper, and therefore, it is impossible to use a plain paper. In addition, in a case of an indirect transferring system, although a plain paper can be used, a toner image formed on the photoreceptor 10 must be transferred onto a plain paper via an intermediate transferring member such as a transferring belt, and therefore, components such as a cooling device for the transferring belt, a zigzag preventing device for the transferring belt are required. Consequently, an image forming apparatus becomes large and a driving system thereof becomes complex.
As a countermeasure, recently, there is proposed a method where a mixed toner which is obtained by mixing a conductive toner and an insulative toner at a predetermined ratio is used (see Japanese Patent Application Laying-Open Nos. 63-135956 and 63-135970).
In Japanese Patent Application Laying-Open No. 63-135956, there is disclosed a method where a photoreceptor including a photoconductive layer is exposed and, at the same time or just thereafter, a conductive toner charged in a negative polarity is brought into contact with the photoreceptor so that an insulative toner charged in a positive polarity is absorbed to the conductive toner by a local Coulomb force is adhered onto a surface of the photoreceptor, whereby a toner image is formed on the photoreceptor by a mixed toner of the conductive toner and the insulative toner. In addition, in Japanese Patent Application Laying-Open No. 63-135970, there is disclosed a method where a mixed toner of an insulative toner and a conductive toner is used as similar to the above, but each of the toner is a magnetic toner.
However, in the former method, since a charged polarity of the insulative toner and a charged polarity of the photoreceptor which is charge-injected by a developing bias through the conductive toner become opposite to each other, a Coulomb force due to an electrostatic latent image formed by a potential well does not act on the insulative toner. Conversely, the insulative toner is adhered by a developing sleeve, and therefore, an amount of the insulative toners which are adhered onto the photoreceptor becomes extremely small, and consequently, an image density becomes low. In addition, in a case of inverted developing, since a surface potential of a non-image portion and a charged polarity of the insulative toner become opposite to each other, there was a disadvantage that a so-called background fog phenomenon where the insulative toner is adhered on the surface of the non-image portion occurs.
In addition, in the latter method, since the insulative magnetic toner and the conductive magnetic toner are both adhered onto the surface of the developing sleeve, it is difficult to form a smooth electric conductive path toward the photoreceptor. Therefore, a charge which is injected into the photoreceptor through the electric conductive path lacks, and therefore, there was a disadvantage that a background fog phenomenon also occurs.